In many animal species, males must compete fiercely with other males for access to mates. As a result, males often develop elaborate traits that will help them conquer other males, or make them more attractive to females. However, sexual competition can also select for traits that are more than just skin-deep. Courtship and mating are complex tasks requiring memory and problem-solving skills, which selection could also act on.

Brian Hollis and Tadeusz Kawecki from the University of Lausanne in Switzerland decided to investigate whether sexual competition influences cognitive performance in fruit flies (Drosophila melanogaster). Fruit flies have relatively simple courtship and mating rituals, but the researchers hypothesized that even in this system, competition for females might select for increased cognitive performance in males. In order to eliminate sexual selection, the authors raised three replicate populations of fruit flies for over 100 generations by randomly pairing single males with single females. In doing this, the authors eliminated all male–male competition, as well as all mate choice. The authors then tested the flies raised in enforced monogamy against males from the original population that had been kept under the naturally polygamous conditions, where males compete with each other to mate with multiple females.

First, the scientists challenged the flies to compete for mating opportunities. They found that when multiple males had to compete over females, the males from the polygamous lines were far more likely to mate successfully with females than the males that came from 100 generations of enforced monogamy. However, there were no differences in locomotion between the flies and no differences between the flies in their mating success when a single male was paired with a single receptive female. The researchers concluded that the monogamous males had no gross abnormalities, but had lost their ability to compete with other males for mates.

In order to identify what these monogamous males might be doing wrong, the scientists put varying numbers of receptive and non-receptive females with either a single monogamous or a single polygamous male. They found that males from the lines of enforced monogamy would waste substantial effort trying to court unreceptive females, while the polygamous males targeted their courtship efforts appropriately. The scientists concluded that over the generations, the monogamous males had lost the cognitive ability to correctly identify the females that would be the most receptive to their amorous advances.

Finally, the researchers asked whether this decline in cognition was specific to courtship, or whether the monogamous males had reduced performance during other cognitive tasks. They trained the flies to associate a specific odour with an unpleasant shock and then challenged the fruit flies to solve a maze where the flies had to chose between the adverse odour and a neutral smell. The researchers found that the monogamous males were worse than the polygamous males at solving the maze. Interestingly, when they tested females from both the monogamous and polygamous lines, the scientists found that females of the two lines were equally capable of solving the maze. This result suggests that the males' decline in cognition is specific to the lack of sexual competition, rather than a result of genetic drift or a decline in overall performance in the absence of mate choice. Thus, it appears that sexual competition selects for cognitive traits, and competition is necessary to keep males sharp.

Similar articles

Other journals from The Company of Biologists

Neuropeptide evolution and function

Neuropeptides are a diverse assemblage of signalling molecules that have key roles in the regulation of behaviour. Understanding the evolutionary relationships and functions of the plethora of neuropeptides has presented a considerable challenge to biologists. Based on presentations and discussions at a Royal Society meeting in 2017, three companion Review articles by Elphick et al., Jékely et al. and DeLaney et al. discuss advances in our knowledge of neuropeptide evolution and function and the techniques that have facilitated progress in this field of research.

The exquisite bright colours of Pachyrhynchus weevils were thought by Alfred Russel Wallace to warn off potential predators, but whether this warning related to their hard exteriors, their spiky legs or some irritating taste had never been tested. Now, a century and half later, a team from Taiwan revisits this question and suggest that hardness itself acts as an effective secondary defence.

In our latest early-career researcher interview, Brooke Flammang, Assistant Professor at the New Jersey Institute of Technology, tells us about her research journey (including writing her Master's thesis in an ambulance while working as a paramedic), the importance of collaboration in integrative biology, and her approach to teaching.

"The paper provided the first quantitative field evidence of the way that animals might gain protection from predation by seeking cover in a group of other similar animals. This protection is known as the dilution effect."

William Foster discusses ‘Evidence for the dilution effect in the selfish herd from fish predation on a marine insect’, the 1981 classic he published in Nature with John Treherne, former JEB Editor-in-Chief.